This invention relates to rolled, extruded or forged products made of a quenched and stretched AlCuMg alloy designed for the manufacture of aircraft structural elements, particularly skin panels and lower wing stringers, and with an improved compromise between mechanical strength, formability, toughness, tolerance to damage and residual stress properties than is available with products according to prior art used for the same application. Designations of alloys and metallurgical tempers are in accordance with the terminology used by the Aluminum Association, and repeated in European standards EN 515 and EN 573.
Wings for high capacity commercial aircraft comprise an upper part (upper wing) composed of a skin made of thick 7150 alloy plates in tempers T651, or 7055 alloy plates in temper T7751 or 7449 alloy plates in temper T7951, and stringers made from profiles of the same alloy, and a lower part (lower wing) composed of a skin made of thick plates made of a 2024 alloy in temper T351 or a 2324 alloy in temper T39, and stringers made from profiles of the same alloy. The two parts are assembled by spars and ribs.
The chemical composition of 2024 alloy according to the terminology used by the Aluminum Association and standard EN 573-3 is as follows (% by weight):
Si less than 0.5 Fe less than 0.5 Cu:3.8-4.9 Mg:1.2-1.8 Mn:0.3-0.9 Cr less than 0.10 Zn less than 0.25 Ti less than 0.15
Different variants have been developed and registered with the Aluminum Association as 2224, 2324 and 2424, particularly with lower contents of silicon and iron. Alloy 2324 in temper T39 was described in Boeing patent EP 0038605 (=U.S. Pat. No. 4,294,625), in which the improvement to the yield stress is obtained by work hardening by a cold rolling pass after quenching. This work hardening tends to reduce the toughness, and the contents of Fe, Si, Cu and Mg are reduced in order to compensate the drop in toughness. Boeing also developed alloy 2034 with composition:
Si less than 0.10 Fe less than 0.12 Cu: 4.2-4.8 Mg:1.3-1.9 Mn: 0.8-1.3 Cr less than 0.05 Zn less than 0.20 Ti less than 0.15 Zr:0.08-0.15
This alloy is described in patent EP 0031605 (=U.S. Pat. No. 4,336,075). Compared with alloy 2024 in temper T351, it has a better specific yield stress due to an increase in the manganese content and the addition of another anti-recrystallizing agent (Zr), and also has improved toughness and fatigue strength.
Alcoa patent EP 0473122 (=U.S. Pat. No. 5,213,639) describes an alloy, recorded by the Aluminum Association as 2524, with composition:
Si less than 0.10 Fe less than 0.12 Cu: 3.8-4.5 Mg: 1.2-1.8 Mn:0.3-0.9
that can also contain an other anti-recrystallizing agent (Zr, V, Hf, Cr, Ag or Sc). This alloy is intended specifically for thin plates for fuselages and its toughness and resistance to crack propagation are improved compared with 2024.
Patent application EP 0731185 made by the applicant relates to an alloy, subsequently registered as 2024A with composition:
Si less than 0.25 Fe less than 0.25 Cu:3.5-5 Mg:1-2 Mn less than 0.55
satisfying the relation: 0 less than Mn-2Fe less than 0.2
The thick plates made of this alloy are tougher and residual stresses are lower, without any loss of other properties.
Alcoa patents U.S. Pat. No. 5,863,359 and U.S. Pat. No. 5,865,914 relate to an aircraft wing with a lower wing made of an alloy with composition:
Cu:3.6-4 Mg:1-1.6 (preferably 1.15-1.5)
Mn:0.3-0.7 preferably 0.5-0.6), Zr:0.05-0.25 and preferably Fe less than 0.07 and Si less than 0.05
with the following properties:
R0.2(LT) greater than 60 ksi (414 MPa) and Kie(L-T) greater than 38 ksiinch (42 MPam),
and a process for manufacturing a lower wing element with R0.2(LT) greater than 60ksi comprising casting of an alloy with the previous composition, homogenization between 471 and 482xc2x0 C., hot transformation at a temperature  greater than 399xc2x0 C.; solution heat treatment above 488xc2x0 C., quenching cold work hardening preferably by more than 9% and stretching by at least 1%.
One essential constraint when constructing new high capacity commercial aircraft is to limit the weight, such that manufacturer specifications impose higher typical stresses for wing panels, which leads to higher minimum values for static mechanical properties and higher damage tolerance for the aluminum alloy products used. The use of work hardened products in temper T39, as recommended in U.S. Pat No. 5,863,359 and U.S. Pat. No. 5,865,914, does give higher yield stresses R0.2, but it also has a number of disadvantages for other working properties that are important in this application. One result is a very small plastic range, in other words the difference between the ultimate stress Rm and the yield stress R0.2, which results in lower cold formability and less resistance to crack propagation under a load with a variable amplitude. The reduction in the rate of crack propagation after a partial overload is less important if the plastic range is small.
Furthermore, large parts must be machined without distortion in thicker plates, which requires better control of residual stresses. However, temper T39 is not particularly advantageous from this point of view.
Therefore the purpose of this invention is to provide AlCuMg alloy products in the work hardened and cold deformed temper, for use in manufacturing aircraft lower wings, and providing a better compromise of all working properties (mechanical strength, rate of crack propagation, toughness, resistance to fatigue and residual stresses) than is possible with similar products according to prior art.
The purpose of the invention is a rolled, extruded or forged product made of an AlCuMg alloy processed by solution heat treatment, quenching and cold stretching, to be used in the manufacture of aircraft structural elements, with the following composition (% by weight):
Fe less than 0.15 Si less than 0.15 Cu:3.8-4.4 (preferably: 4.0-4.3) Mg:1.0-1.5 Mn: 0.5-0.8 Zr:0.08-0.15
other elements:  less than 0.05 each and  less than 0.15 total with a ratio Rm(L)/R0.2 (L) of the ultimate tensile strength in the L direction to the yield stress in the L direction exceeding 1.25 (and preferably exceeding 1.30).
Another purpose is a rolled product (plate) with the same composition and between 6 and 60 mm thick and with at least the following groups of properties in the quenched and stretched temper:
a) Ultimate tensile strength Rm(L) greater than 475 MPa and yield stress R0.2(L) greater than 370 MPa
b) Plastic range Rm-R0.2 in the L and TL directions  greater than 100 MPa.
c) Critical intensity factor (L-T direction) Kc greater than 170 MPam and Kco greater than 120 MPam (measured according to ASTM standard E 561 on notched test pieces sampled at a quarter thickness with parameters B=5 mm, W=500 and 2B0=165 mm)
d) Crack propagation rate (L-T direction) da/dn, measured according to ASTM standard E 647 on notched test pieces sampled at a quarter thickness with parameters W=200 mm and B=5 mm) less than 10xe2x88x924 mm/cycle for xcex94K=10 MPam  less than 2.5 10xe2x88x924 mm/cycle for xcex94K=15 MPam and  less than 5 10xe2x88x924 mm/cycle for xcex94K=20 MPam
This plate also has residual stresses such that the deflection f measured in the L and TL directions after machining a bar supported on two supports separated by a length 1 to its mid-thickness, is such that:
f less than (0.14 l2)/e, where f is measured in microns, e is the thickness of the plate and l is the length measured in mm.
Another purpose of the invention is a process for manufacturing a rolled, extruded or forged product comprising the following steps:
cast a plate or billet with the indicated composition,
homogenize this plate or billet between 450 and 500xc2x0 C.,
hot transformation, and possibly cold transformation, until the required product is obtained,
solution heat treatment at a temperature of between 480 and 505xc2x0 C.,
quench in cold water,
cold stretching to at least 1.5% permanent deformation,
natural aging under ambient conditions.